Membership.ValidateUser(long companyId, string username, string password) overload

Question

Trying to get the method to check this way:

 Membership.ValidateUser(long companyId, string username, string password)

As an overload. I already have a class that implements the MembershipProvider, MembershipUser, and RoleProvider classes (with overrides), but I can only cast the result of the Membership.ValidateUser return into what I need.

Essentially, what I want to do is have the Username + Password combo NOT be Unique across the database UNLESS there is a CompanyId provided as well. The other requirement is that I cannot join the Username + CompanyId in the db (a la JSMITH&012345) to make the thing unique.